Plasma spray is used in manufacturing operations in aerospace, energy, engine, electronic, and biomedical because it can economically produce engineered coatings that protect against wear, reduce friction, and reduce corrosion. The plasma spray coatings are also particularly suited to protect underlying metals against high temperature environments, such as in jet engines. Either ceramic or metallic coatings can be formed with plasma spray. However, up until now parts coated with plasma spray have varied substantially from each other. The industry has therefore not been able to reliably design parts with tightly specified engineered coating structures or to reproduceably provide a desired porosity, crack density, and grain structure. Nor has the industry been able to consistently provide sufficient control over sprayed layers so different materials could be provided with tightly specified properties and thicknesses or so layers with varying composition were deposited. Looser specifications than desired have been needed in production or a substantial fraction of coatings have had to be reworked. And no tool has been available that provides improved control over these coating parameters while providing a high deposition rate to reduce cost of sprayed layers.
One scheme to improve plasma spray process was disclosed in an article, “Feedback Control of the Subsonic Plasma Spray Process: Controller Performance,” Fincke, J. R., et. al., Proceedings of the 8th National Thermal Spray Conference, September 1995 Houston, pp 117-122, in which the author demonstrated the ability to independently control both the particle velocity and the temperature of the particles coming from a plasma spray torch. In this article, particle temperature was measured and the measurement fed back to adjust torch current. The particle velocity measurement was fed back to adjust torch gas flow rate.
Another scheme to improve plasma spray process was disclosed in an article, “Intelligent Processing of Materials for Thermal Barrier Coatings,” by Y. C. Lau, et al, TBC Workshop 1997, sponsored by the TBC Interagency coordination Committee, NASA Lewis Research Center, in which the authors provided an interaction matrix and coordinated manipulation of torch current and plasma gas flow inputs to control both temperature and velocity.
While both approaches improved control over plasma spray coatings, wide variation from part to part and even during the process of spraying a single part remained. Thus, a better system for controlling plasma spray is needed, and this solution is provided by the following description.